SQL GROUP BY Explained

Quick answer

GROUP BY is used in SQL to combine rows that share the same value in one or more columns. Once rows are grouped, you usually use aggregate functions like COUNT(), SUM(), AVG(), MIN(), or MAX() to summarize each group.

In beginner terms, GROUP BY answers questions like:

• How many employees are in each department?
• What is the average salary in each city?
• How much total revenue came from each product category?

In this guide

Sample table

To keep the examples simple, imagine this employees table:

What GROUP BY does

GROUP BY takes rows that have the same value in a chosen column and puts them into buckets.

For example, if you group by department, all Sales rows go into one bucket, all IT rows go into one bucket, and all HR rows go into one bucket.

SELECT department
FROM employees
GROUP BY department;

That would return one row per department.

Count rows in each group

One of the most common uses of GROUP BY is counting how many rows fall into each category.

SELECT department, COUNT(*) AS employee_count
FROM employees
GROUP BY department;

This gives you one row per department, along with the number of employees in each department.

Get totals for each group

You can also total numeric values within each group using SUM().

SELECT department, SUM(salary) AS total_salary
FROM employees
GROUP BY department;

This returns the total salary cost for each department.

Get averages for each group

Averages are another common use of grouped summaries.

SELECT department, AVG(salary) AS avg_salary
FROM employees
GROUP BY department;

This shows the average salary in each department.

Group by multiple columns

You can group by more than one column if you want more specific buckets.

SELECT department, city, COUNT(*) AS employee_count
FROM employees
GROUP BY department, city;

This creates groups for each department-and-city combination instead of grouping only by department.

Using HAVING with GROUP BY

WHERE filters rows before grouping. HAVING filters grouped results after grouping.

SELECT department, COUNT(*) AS employee_count
FROM employees
GROUP BY department
HAVING COUNT(*) > 1;

This returns only departments with more than one employee.

When to use GROUP BY

Use GROUP BY when you want one result per category instead of one big result for the whole table.

Common mistakes

• Using a normal column with an aggregate function without adding that normal column to GROUP BY
• Confusing WHERE and HAVING
• Grouping by too many columns and accidentally creating tiny buckets
• Forgetting that each group returns one summary row
• Using GROUP BY when a normal SELECT would have been enough

Practice questions

Bottom line

GROUP BY is one of the most important SQL skills because it lets you summarize data by category.

Once you understand how to group rows and apply aggregate functions, you can answer a huge number of real reporting and analysis questions.

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